The present invention relates generally to magnetic field sensors, and specifically to sensors formed from thin films.
A method well known in the art for measuring magnetic field is to utilize the Hall effect, which generates an electrical potential in a conductive material. The potential generated is directly dependent on an electric current flowing in the material and on the magnetic field perpendicular to the current.
FIG. 1 is a diagram of a ferromagnetic conductor 10, having a general rectangular film-like shape, as is known in the art. Conductor 10 has a current I flowing between faces 12 and 14 of the conductor, and there is a magnetic field B applied through faces 16 and 18 of the conductor, causing a magnetization M in the conductor. Face 16 (or 18) defines a plane of conductor 10. Field B acts on moving current carriers in conductor 10 to generate a Hall voltage VH between faces 20 and 22 of the conductor. In general:
VH=Ixc2x7(Rnxc2x7B+Rexc2x7M)xe2x80x83xe2x80x83(1)
where Rn is a first constant, termed the normal Hall coefficient, and Re is second constant, termed the extraordinary Hall coefficient, for conductor 10.
The normal Hall coefficient, Rn, represents the effect of Lorentz forces on the current carriers in conductor 10. The extraordinary Hall coefficient Re, characteristic of conductors which are ferromagnetic, represents the effect of scattering of electrons in the presence of magnetic polarization.
In bulk ferromagnetic materials, Re can be much larger than Rn, so that for values of B lower than those saturating conductor 10, equation (1) can be rewritten as:
VH=Ixc2x7Rexc2x7M=Ixc2x7Rexc2x7"khgr"Bxe2x80x83xe2x80x83(2)
where "khgr" is an effective susceptibility, dependent on the geometry and composition, of conductor 10.
FIG. 2 is a graph, as is known in the art, illustrating a relation between a measured Hall resistance RHall and magnetic field B at room temperature, for a nickel film having a thickness of 100 nm. RHall corresponds to the term Rexc2x7"khgr"B of equation (2). The graph shows that in a region between xe2x88x920.3 T and 0.3 T RHall varies approximately linearly with magnetic field, that the slopes of the linear sections,             ⅆ              R        Hall                    ⅆ      B        ,
are approximately 30 mxcexa9/T, and that there is a hysteresis of approximately 1.1xc2x710xe2x88x922 T. The term       ⅆ          R      Hall            ⅆ    B  
is termed the field sensitivity, F, of the film.
From equation (2), and the definitions Hall resistance RHall and of field sensitivity F,                                           R            Hall                    =                                    R              e                        ·            χ            ·            B                          ,                              so            ⁢                          xe2x80x83                        ⁢            that            ⁢                          xe2x80x83                        ⁢                                          ⅆ                                  R                  Hall                                                            ⅆ                B                                              =                      F            =                                          R                e                            ·              χ                                                          (        3        )            
Returning to FIG. 1, a sensitivity S of conductor 10, when it is used as a ferromagnetic Hall sensor, may be defined as             V      H        B    ,
so that from equations (2) and (3):                     S        =                                            V              H                        B                    =                                    I              ·              χ              ·                              R                e                                      =                          I              ·              F                                                          (        4        )            
For Hall sensors which are not ferromagnetic, such as semiconductors, equation (1) becomes:
VH=Ixc2x7Rnxc2x7Bxe2x80x83xe2x80x83(5)
A sensitivity S, from equation (5), may be written as:                     S        =                                            V              H                        B                    =                      I            ·                          R              n                                                          (        6        )            
Hall sensors using both ferromagnetic and non-ferromagnetic materials, the latter typically being semiconductors, are known in the art. Typically, an effective Hall coefficient for a bulk ferromagnetic, "khgr"xc2x7Re, is significantly smaller than the Hall coefficient, Rn, of a semiconductor.
A relationship between the extraordinary Hall coefficient Re and the resistivity xcfx81 of ferromagnets is known in the art. The relationship is of the form:
Rexe2x88x9dxcfx81nxe2x80x83xe2x80x83(7)
where n is a constant.
The value of n in equation (7) is dependent on the composition of the ferromagnet, and typically lies in a range between 1 and approximately 4. Variations of xcfx81 of the order of ten percent are typically produced by doping or temperature changes.
Ferromagnetic conductors such as conductor 10 may be implemented in one of two anisotropic forms. Planar anisotropy, wherein a direction of easy magnetization of the conductor lies in the plane of the conductor, and perpendicular anisotropy, wherein the direction of easy magnetization is perpendicular to the plane of the conductor. As known in the art, both forms exhibit some hysteresis, although the hysteresis of conductors which have perpendicular anisotropy is typically larger than the hysteresis of planar anisotropy conductors. Films with reduced thickness typically have planar anisotropy, although perpendicular anisotropy is known in such films, and in certain alloys, such as Coxe2x80x94Pt, Coxe2x80x94Cr, and Coxe2x80x94Crxe2x80x94Ta. The implementation of conductor 10 as a planar or as a perpendicular anisotrope is typically a function of how conductor 10 is formed, and the composition of the conductor, as is known in the art.
An article titled xe2x80x9cSpin-dependent scattering in weakly coupled nickel films,xe2x80x9d by Gerber et al., in Europhysics Letters, 49(3), (2000), which is incorporated herein by reference, describes a process of forming thin films from ferromagnetic materials. Initially, as films are formed on an insulating substrate by electron beam evaporation or radio-frequency sputtering, islands of metal build on the substrate. The process of island formation continues until a conductance percolation threshold is achieved, wherein there is a conducting path through the film between weakly coupled islands of the film. If deposition of the ferromagnetic material continues, a ferromagnetic percolation threshold is achieved, wherein the film becomes ferromagnetic. Between the conductance percolation threshold and the ferromagnetic percolation threshold the film behaves substantially as a super-paramagnetic.
It will be appreciated that the conductance percolation threshold may be evaluated for substantially any film of conducting material, for example, by determining during formation of the film a point at which the film begins to conduct. Similarly, the ferromagnetic percolation threshold may be evaluated for substantially any film of ferromagnetic material, for example, by determining during formation of the film the point at which the film begins to behave as a ferromagnet. Alternatively or additionally, the conductance percolation threshold may be evaluated by indirect measurement of conductance of the film, and the ferromagnetic percolation threshold may be evaluated by determining the presence of hysteresis in the film. Other methods for determining both thresholds will be apparent to those skilled in the art.
U.S. Pat. No. 4,393,427, to Sakurai, whose disclosure is incorporated herein by reference, describes a magnetic detecting head comprising a Hall element. The Hall element is an amorphous ferromagnetic film comprising a rare earth/transition metal alloy, and having a thickness of 200 nm or more. U.S. Pat. No. 4,420,781, to Sakurai, whose disclosure is incorporated herein by reference, also describes a magnetic detecting head comprising a Hall element having a thickness of about 150 nm.
U.S. Pat. No. 5,206,590, to Dieny, et al., whose disclosure is incorporated herein by reference, describes a magnetoresistive sensor comprising a first ferromagnetic film and a second ferromagnetic film separated by a non-magnetic metallic film. The sensor uses a xe2x80x9cspin valvexe2x80x9d effect occurring between the two ferromagnetic films, wherein the resistance between two uncoupled ferromagnetic layers varies as the cosine of the angle between magnetizations of the two layers.
U.S. Pat. No. 5,361,226, to Taguchi, et al., whose disclosure is incorporated herein by reference, describes a magnetic memory comprising a ferromagnetic film which has perpendicular anisotropy. One of the embodiments of the memory comprises a film having a thickness of 50 nm.
U.S. Pat. No. 5,617,071, to Daughton, whose disclosure is incorporated herein by reference, describes a magnetoresistive layered structure having a plurality of layers of ferromagnetic films. Providing multiple layers increases a xe2x80x9cgiant magnetoresistivexe2x80x9d (GMR) response of the structure when it is used as a field sensor.
U.S. Pat. No. 5,652,445, to Johnson, whose disclosure is incorporated herein by reference, describes a hybrid Hall device. The device comprises a ferromagnetic film which is over-layered on a portion of a conductive layer. The device generates an electric signal responsive to a fringe magnetic field, from the ferromagnetic film, through the conductive layer.
Notwithstanding the systems described above, there is a need for a relatively simple magnetic field sensor which may be simply and robustly fabricated.
It is an object of some aspects of the present invention to provide a method and apparatus for sensing a magnetic field.
In some preferred embodiments of the present invention, a thin film comprising ferromagnetic material is implemented to have an enhanced extraordinary Hall effect. The enhancement is produced by forming the film so that electron scattering in the film is significantly larger than electron scattering of the bulk ferromagnetic material. Forming the film in this manner increases the electrical resistivity of the film, and thus increases the extraordinary Hall effect. The enhanced extraordinary Hall effect of the film enables the film to be used as an efficient magnetic field sensor.
The thin film is formed on a substrate, which acts as a mechanical support for the film, and the substrate may be any material which does not interfere with the operation of the film. The film is formed to have a film thickness no greater than a threshold thickness at which the resistivity is substantially equal to 150% of a bulk resistivity of the material thus achieving a corresponding enhanced extraordinary Hall effect. The enhanced extraordinary Hall effect of the thin film provides a corresponding enhancement in sensitivity of the sensor compared to unenhanced sensors, and using resistivity as a measure of the efficacy of the thin film substantially simplifies the process of fabricating the sensor.
The thin film is preferably formed by sputtering or electron beam deposition, or alternatively by any other method known in the art for producing thin films. The thin film may be implemented in a number of different forms:
As a substantially homogeneous film of ferromagnetic material, such as cobalt or nickel.
As a ferromagnetic matrix comprising insulating particles embedded within the matrix.
The thin film may also be implemented as a combination of the above forms.
In an alternative preferred embodiment of the present invention, the thin film is formed from weakly coupled ferromagnetic clusters. The thin film is implemented so that a thickness of the film is greater than that needed to achieve a conductance percolation threshold, but less than the thickness for the film to behave as a ferromagnetic, i.e., achieve a ferromagnetic percolation threshold. Within these thicknesses, the film is formed to have an increased resistivity, and thus an enhanced extraordinary Hall coefficient, compared to the bulk resistivity of the ferromagnetic.
In another alternative preferred embodiment of the present invention, the thin film is formed as a film comprising ferromagnetic particles embedded in a matrix of non-magnetic material. The thin film is implemented so that a volume ratio of the ferromagnetic particles to the non-magnetic material lies between a lower and an upper value. The lower value is approximately 0.05%, the upper value corresponds to a volume ratio for the ferromagnetic percolation threshold for the thin film, which is typically in a range between approximately 15% and approximately 100%. Within these values, the film may have an enhanced extraordinary Hall coefficient, compared to the extraordinary Hall coefficient for the bulk ferromagnetic. This occurs, for example, for cobalt grains embedded in a platinum matrix, compared to bulk cobalt.
Thin films formed as described above have a number of desirable properties enabling them to operate efficiently as magnetic field sensors:
At temperatures approximating room temperature, the thin film behaves as if it had planar or no anisotropy, i.e., the thin film exhibits little or no hysteresis.
The signal generated by the thin film is highly linear with magnetic field.
The sensitivity of the thin film is orders of magnitude larger than that of bulk ferromagnets, and may be comparable to or higher than the sensitivity of semiconductor Hall devices.
In some preferred embodiments of the present invention, a protective layer is formed over the thin film. The protective layer preferably comprises any insulating material, such as silicon dioxide, which has properties enabling it to protect the thin film.
In a further alternative preferred embodiment of the present invention, the thin film, implemented by one of the methods described above, is fabricated to have substantially perpendicular anisotropy. The thin film has substantial hysteresis, and may thus be used as a memory device.
There is therefore provided, according to a preferred embodiment of the present invention, a method for producing a material with an enhanced extraordinary Hall coefficient, including:
determining a conductance percolation threshold for the material;
determining a first value of a characteristic of the material at the conductance percolation threshold;
determining a ferromagnetic percolation threshold for the material;
determining a second value of the characteristic of the material at the ferromagnetic percolation threshold; and
fabricating the material so that the characteristic of the material lies between the first and the second values of the characteristic.
Preferably, the material includes a ferromagnet having weakly coupled grains.
Preferably, the method further includes forming the material as a thin film, wherein the characteristic of the material includes a thickness of the thin film.
Further preferably, the first value of the thickness is approximately 3 nm, and the second value of the thickness is approximately 100 nm, and the thin film consists of nickel.
Preferably, fabricating the material includes setting the thickness so that the thin film has substantially no hysteresis.
There is further provided, according to a preferred embodiment of the present invention, a magnetic field sensor, including:
an insulating substrate;
a conductive thin film deposited on the substrate, the thin film including a material having an extraordinary Hall coefficient, the thin film having a thickness lying between a first thickness of the thin film at which a conductance percolation threshold for the material occurs and a second thickness of the thin film at which a ferromagnetic percolation threshold occurs; and
conductors coupled to the thin film for injecting a current into the film and measuring a voltage generated across the thin film responsive to the injected current.
Preferably, the thin film includes a ferromagnet having weakly coupled grains.
Further preferably, the first thickness is approximately 3 nm, and the second thickness is approximately 100 nm, and the ferromagnet consists of nickel.
Preferably, the thickness is set so that the conductive thin film has substantially no hysteresis.
There is further provided, according to a preferred embodiment of the present invention, a magnetic field sensor, including:
an insulating substrate;
a conductive thin film deposited on the substrate, the thin film including a material having an extraordinary Hall coefficient, the thin film having a resistivity and having a film thickness no greater than a threshold thickness at which the resistivity is substantially equal to 150% of a bulk resistivity of the material; and
conductors coupled to the thin film for injecting a current into the film and measuring a voltage generated across the thin film responsive to the injected current.
Preferably, the thin film includes a substantially homogeneous film consisting of a ferromagnet.
Preferably, the film thickness is less than approximately 10 nm.
Alternatively or additionally, the film thickness is less than approximately 6 nm.
Preferably, the film thickness is no greater than the thickness at which the resistivity is substantially equal to 200% of the bulk resistivity.
Alternatively or additionally, the film thickness is no greater than the thickness at which the resistivity is substantially equal to 250% of the bulk resistivity.
Preferably, the film thickness is set so that the conductive thin film has substantially no hysteresis.
Preferably, the film is implemented to have substantially parallel anisotropy.
There is further provided, according to a preferred embodiment of the present invention, a magnetic field sensor, including:
an insulating substrate;
a conductive thin film deposited on the substrate, the thin film consisting of a material having an extraordinary Hall coefficient and including an array of insulating inclusions within a ferromagnetic matrix, the thin film having a resistivity and having a volume ratio of the insulating inclusions to the ferromagnetic matrix no less than a threshold volume ratio at which the resistivity is substantially equal to 150% of a bulk resistivity of the material; and
conductors coupled to the thin film for injecting a current into the film and measuring a voltage generated across the thin film responsive to the injected current.
Preferably, the volume ratio of the insulating inclusions to the ferromagnetic matrix is sufficiently small so that the thin film is above a conductance percolation threshold.
Preferably, the insulating inclusions include silicon dioxide, and the ferromagnetic matrix consists of nickel, and the volume ratio is less than or equal to approximately 100%.
Preferably, the volume ratio is set so that the conductive thin film has substantially no hysteresis.
There is further provided, according to a preferred embodiment of the present invention, a memory, including:
a substrate; and
a conductive thin film deposited on the substrate so as to be substantially perpendicularly anisotropic, the thin film consisting of a material having an extraordinary Hall coefficient and having a resistivity and a film thickness no greater than a threshold thickness at which the resistivity is substantially equal to 150% of a bulk resistivity of the material.
There is further provided, according to a preferred embodiment of the present invention, a magnetic field sensor, including:
an insulating substrate;
a conductive thin film deposited on the substrate, the thin film including ferromagnetic particles within a conducting non-ferromagnetic matrix, a volume ratio of the ferromagnetic particles to the matrix lying between a lower volume ratio approximately equal to 0.05% and an upper volume ratio at which a ferromagnetic percolation threshold is achieved; and
conductors coupled to the thin film for injecting a current into the film and measuring a voltage generated across the thin film responsive to the injected current.
Preferably, the upper volume ratio lies in a range between approximately 15% and approximately 100%.
Further preferably, the ferromagnetic particles include cobalt particles, and the conducting non-ferromagnetic matrix includes platinum.
There is further provided, according to a preferred embodiment of the present invention, a method for producing a thin film with an enhanced extraordinary Hall coefficient, including:
determining a bulk resistivity for a material comprising the thin film; and
fabricating the thin film so that the fabricated thin film has a resistivity and a film thickness no greater than a threshold thickness at which the resistivity is substantially equal to 150% of the bulk resistivity.
Preferably, fabricating the thin film includes fabricating the film thickness so that the thin film has substantially no hysteresis.
There is further provided, according to a preferred embodiment of the present invention, a method for producing a thin film with an enhanced extraordinary Hall coefficient, including:
determining a bulk resistivity for a ferromagnetic material included in the thin film; and
fabricating the thin film as an array of insulating inclusions within a matrix including the ferromagnetic material, so that the fabricated thin film has a resistivity and so that a volume ratio of the insulating inclusions to the ferromagnetic matrix is no less than a threshold volume ratio at which the resistivity is substantially equal to 150% of the bulk resistivity of the ferromagnetic material.
Preferably, the volume ratio of the insulating inclusions to the ferromagnetic matrix is sufficiently small so that the thin film is above a conductance percolation threshold.
Preferably, the insulating inclusions consist of silicon dioxide, and the ferromagnetic matrix includes nickel, and the volume ratio is less than or equal to approximately 100%.
Preferably, the volume ratio is set so that the thin film has substantially no hysteresis.
There is further provided, according to a preferred embodiment of the present invention, a method for producing a thin film with an enhanced extraordinary Hall coefficient, including:
fabricating the thin film as an array of ferromagnetic particles within a conducting non-ferromagnetic matrix;
adjusting a volume ratio of the ferromagnetic particles to the matrix to be between a lower volume ratio approximately equal to 0.05% and an upper volume ratio at which a ferromagnetic percolation threshold is for the thin film is achieved.
Preferably, the upper volume ratio lies in a range between approximately 15% and approximately 100%.
Preferably, the ferromagnetic particles include cobalt particles, and the conducting non-ferromagnetic matrix includes platinum.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: